An SCR system that uses a selective reduction catalyst is developed as an exhaust gas purification system to purify NOx contained in an exhaust gas of a diesel engine
This SCR system supplies urea water, which is retained or pooled in a urea water tank, to an upstream exhaust gas of an SCR device to hydrolyze the urea water with the heat of the exhaust gas and generate ammonia. The SCR system then uses the ammonia to reduce NOx with a catalyst inside the SCR device, thereby purifying the exhaust gas. The urea water is sprayed from a dosing valve disposed upstream of the SCR device to supply the urea water to the upstream exhaust gas of the SCR device.
Feeding of the urea water to the dosing valve is carried out by a supply module that includes a supply module pump (SM pump), a urea water pressure sensor and other components. The supply module is connected to the urea water tank via a draw-in line (suction line), and supplies the urea water, which is drawn in from the urea water tank via the draw-in line, to the dosing valve through a pressurized urea water feed line extending between the supply module and the dosing valve. The dosing valve is controlled by a DCU (dosing control unit) such that opening and closing of the dosing valve is controlled in response to a detection value of a NOx sensor disposed downstream of the SCR device to adjust an amount of urea to be sprayed (urea spray volume).